1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system which transports optical signals over a long distance.
2. Description of the Related Art
As a core component of today's information and communication infrastructures, optical network systems are expected to provide more sophisticated services in wider geographical areas. Rapid technology developments have thus been made in this field to meet the future needs in advanced information-age societies. Of particular interest in recent years is repeaterless optical transmission systems. While transoceanic undersea cable systems require intermediary repeaters to compensate for their fiber-opitc optic cable losses, repeaterless optical transmission techniques can be used in other long-haul applications such as continent-to-island or island-to-island communications systems. There is an increasing demand for reliable repeaterless systems because they are advantageous in both construction and service costs.
Wavelength division multiplex (WDM) systems are widely deployed as the mainstream technology of modern optical communication. WDM enables multiple signals to be carried by a single fiber-optic cable, multiplexing them in the wavelength domain. Besides transporting main information signals of 2.4 to 40 Gb/s, WDM systems have a monitoring signal channel of 1.5 to 150 Mb/s, which is called “Optical Supervisory Channel (OSC).” OSC signals are used to set up a system and monitor its operation status. More specifically, optical amplifiers in a transmission system are configured, monitored, and controlled by using an OSC channel. Another purpose of OSC is to detect a failure of optical transmission lines. Because of such usage, OSC signals do not pass through erbium-doped fiber amplifiers (EDFA) employed in many WDM systems (that is, EDFA is for main signals only). Also, because of their nature as the vehicle of control information, the OSC channel is given a relatively lower optical power level in order not to interfere with the main signals.
One solution to extend the cable length without using repeaters is to use a Raman amplifier. Raman amplifiers are known as an optical amplification technique suitable for long-distance, high-speed wideband optical transmission. They uses a physical phenomenon called stimulated Raman scattering. When a light enters into a substance that is vibrating, a part of the light is scattered with a shift of wavelength due to the vibration of that substance. With a sufficiently intense pump wave co-launched into a fiber-optic transmission line, the signal light gains energy from that pump wave, which accomplishes optical amplification. The peak of the Raman gain is obtained when the signal light has a wavelength offset of about 100 nanometers (nm) on the longer wave side. This means that a pump beam amplifies a signal wave whose wavelength is about 100 nm longer than the pump beam wavelength. For example, a 1450-nm pump beam is required to amplify a 1550-nm signal wave.
The above-described WDM transmission and Raman amplification techniques have been an active research area, and there are several proposals for Raman amplifier-based systems with a wider usable gain bandwidth. One such system is disclosed in the Unexamined Japanese Patent Publication No. 2002-229084, particularly in paragraphs 0019 to 0021 and FIG. 1.
In some conventional optical transmission systems, their OSC channel is assigned a relatively shorter wavelength band in the spectrum. This is applied to short-haul systems, as well as to long-distance systems that employ repeaters at short intervals. OSC signals in such systems are placed in a wavelength range of, for example, around 1510 nm, when main signals are arranged in the C-band (1535 to 1561 nm).
In long-haul repeaterless systems, on the other hand, their OSC channel is assigned a wavelength of 1570 to 1580 nm, where the fiber loss is lowest. Such systems have to make optical signals propagate over a distance of as much as 250 km, and an even longer haul is required in some of them. OSC signals are also supposed to reach the remote end even if the upstream optical transmitter amplifier and downstream Raman pump light source are both shut down, because, as mentioned above, they are necessary for the network operator to administrate the system. In other words, OSC signals have to save their power as much as possible during their long journey on the transmission line. This is why conventional systems allocate a lowest-loss wavelength band to their OSC channel.
Another attribute of repeaterless systems is their large power of information-carrying signals and Raman pump beams for the purpose of long-distance transport. More specifically, an optical amplifier at the sending end outputs one or more watts of power, which is more than six times as high as those in typical terrestrial systems. Likewise, a downstream Raman pump light source generates a beam of one to two watts, which is three to five times as high as those in typical terrestrial systems.
As can be seen from the above explanation, conventional repeaterless systems are designed to have an OSC channel with a wavelength that is longer than main signal wavelengths, as well as to produce high power beams with their optical amplifiers and Raman pump beam sources. Such system design, however, has the following problems:
(1) OSC Errors Caused by Amplifier Start-up
Upon start-up, the system enables its upstream optical amplifier to output optical signals, which causes Raman amplification effects on the OSC signal. In that transient period, a sudden increase in the optical amplifier power produces an abrupt Raman gain variation, and the consequent change in the OSC signal level could be too large for an OSC circuit at the receiving end to follow, thus resulting in an OSC signal error.
(2) OSC Errors Caused by Pump Light Source Start-up
Similar to the above problem (1), a sudden start-up of a downstream Raman pump light source causes an abrupt Raman gain variation, and the resulting change in the OSC signal level could be too large for the OSC receiver to follow, ending up with an OSC signal error.
(3) Lack of Stability in Recovery from APSD
High-power optical transmission systems have an automatic power shutdown (APSD) function, which prevents the human body from being exposed to hazardous light beams by automatically turns off optical amplifiers and other high-power light sources upon detection of a failure. When the system tries to recover from shutdown, an OSC error could happen in a transient period of that process just because of the reasons described in (1) and (2). This OSC error triggers the APSD mechanism, and the system should follow the same process again and again.